add missing includes for freebsd

every time I use errno I forget that it requires an include lest it
fails on freebsd.

also add string.h for strerror for good measure

src/linuxprocmapsparser.cpp

@@ -11,6 +11,8 @@
 #include <QFile>
 #include <QRegularExpression>
 
+#include <errno.h>
+#include <string.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>